The invention relates to an RC oscillator having a controllable oscillation frequency, comprising: a series arrangement of a capacitor and resistor, and a signal source for supplying a step voltage signal to the series arrangement, the oscillation frequency in operation being dependent upon the time for charging the capacitor has been charged to a given voltage.
The invention further relates to an electric toaster having an electric heating element, means for connecting the heating element to an electric power source, and an adjustable timer for disconnecting the heating element from the electric power source after an adjustable time interval, the adjustable timer including said RC oscillator.
Such an RC oscillator and toaster are known from, for example, U.S. Pat. No. 5,402,708. This known toaster has a timer by means of which the toasting time can be set. The timer comprises an RC oscillator and a divider circuit which divides the comparatively high oscillation frequency of the RC oscillator to a signal having a low frequency and a period whose length is suitable for toasting. The generation of the oscillation frequency is based on the application of a step voltage to a series arrangement of a resistor and a capacitor and on subsequently waiting until the voltage across the capacitor exceeds a given threshold voltage. In the known RC oscillator the resistance is varied by means of a potentiometer so as to enable the period of the oscillation frequency to be controlled. The threshold voltage is exceeded sooner and the period becomes smaller according as the resistance decreases. Since the resistance of a potentiometer generally has a fairly large tolerance there is a comparatively large spread in the frequency generated by the RC oscillator. If such a potentiometer is now used in the timer of a toaster the toasting time obtained for a given setting of the control knob or slide of the potentiometer will differ from toaster to toaster.